The invention herein relates to adhesives. More particularly it relates to hot melt adhesives.
Wax based hot melt adhesives have been used for many years. It has also been common to incorporate into such adhesives a halogen donor (usually a donor of chlorine or bromine) to impart to flame retardancy to the adhesive. Typical hot melt adhesives have, however, suffered from several major limitations.
1. Hot melt adhesives containing flame retardant halogen chemicals (particularly chlorine chemicals) are unstable when held at high temperatures for extended periods because of dehydrohalogenation. For instance, adhesives held in the molten state at temperatures of about 300.degree.-350.degree. F. (150.degree.-175.degree. C.) for periods of more than six hours change from a molten to a gelled state and thus cannot be used as hot melts. This property severely limits the ability of the hot melt user to maintain large quantities of molten adhesive for extended periods and thus severely shortens the length of coating runs which can be obtained.
2. Among the most widely used flame retardants have been the chlorinated biphenyls. These chemicals have recently been identified as probable carcinogens and have been banned from use in many localities. Thus, many of the prior art hot melt adhesives are no longer available.
3. While flame retardancy was a property incorporated into many prior art hot melt adhesives, few if any contained components intended to reduce smoke generation. In view of many current environmental and occupational health standards, smoke suppression has become an important factor in the formulation of hot melt adhesives.
4. Prior art hot melt adhesives have a definite "tack" or stickiness at room temperature. When these have been used to coat sheet materials which are commonly rolled for storage or shipment (and are therefore referred to as "roll goods") it has been necessary to take steps to prevent the sticky adhesive on one layer of the roll from adhering to the sheet which forms the next layer of the roll. Such means have included incorporating paper separators as part of the roll and/or dusting the sheet surface with materials such as talc. This of course adds substantial additional fabrication and material costs as well as incorporating into the roll goods materials such as separating paper and talc which are not needed for the end use of the roll goods and therefore must be removed and discarded by the user.
5. In many instances, hot melt adhesives are applied to sheets to form adhesive jacketing materials which are placed as facings for building materials and thermal insulations. The jacketed products are frequently used outdoors or in other environments where they are subjected to humidty and/or low temperatures. Heretofore, however, the hot melt adhesives themselves could not serve as vapor barriers or provide resistance to "cold cracking" in low temperatures. Thus vapor barriers had to be provided by additional coatings, jacketings, etc., obviously at additional cost. Care also had to be taken to provide supplemental adhering means if low temperatures were expected. Alternatively, cold cracking often caused hot melt adhesives to be rejected for use in many applications where they would otherwise have been quite suitable.
6. Finally, many hot melt adhesives can be applied only as continuous films or sheets to the substrate on which they are coated. They are not amenable to being applied in discrete segments, as in the form of small discrete dots or lines of adhesive on a substrate. Thus, where discrete areas of adhesive would provide adequate adhesive strength to bond the substrate to another material, the required use of a continuous film of adhesive represents unnecessary added cost and waste of material.